In pulping technology, the wood feedstock is subjected to a cooking treatment process with chemical agents, known as white or green liquor, to remove lignin and hemicellulose, thereby producing a cellulosic pulp. Thanks to the high reactivity of the chemical agents, the cooking treatment is typically conducted in pressurized cooking reactors at moderate temperature and pressure, wherein pressurized steam is used mainly as a heating means. After the cooking treatment, the cellulosic pulp, which is a high consistency suspension of solid cellulosic fibers, is flashed in a blow tank to reduce the pressure to about atmospheric pressure.
Fardim, Pedro, “Chemical Pulping Part 1, Fiber Chemistry and Technology”, Second Edition, Papermaking Science and Technology, 2011, pag. 288-289 (“Fardim”), reports an example of timing and process conditions in a conventional batch kraft cooking system. FIG. 92 illustrates temperature and pressure time profiles. The process temperature is raised to about 175° C. in about 2 hours, then cooking occurs for a cooking time of 45 minutes at a cooking pressure of about 8 bar. Heating is provided by steam at a pressure up to 12 bar, and it is stopped during the cooking phase. After the cooking step, the pulp is blown down in a blow tank. Chips are disintegrated to fibers during the blow, in the blow line, and on the entry to the blow tank through the shearing action caused by turbulent flow and flashing of steam. An example of a blow tank is provided in FIG. 93 in Fardim. The blow tank is equipped with a cyclone separator to allow fiber-free steam to flow to the flash steam condensing system. The blow tank is a large vessel, with standard volume ranging from 100 m3 to 900 m3, to take into account the steam expansion during the blow. The blow tank has a circular shape, with an outlet for pulp discharge at the bottom end and an outlet for flash gas at the top end. The pulp is fed through a blow inlet horizontally located in the upper part of the blow tank.
The working principle of a blow tank, also known as a blow cyclone or pressure cyclone, may be found in Lonnberg, Bruno, “Mechanical Pulping”, Second Edition, Papermaking Science and Technology, 2009, pag. 200 (“Lonnberg”). FIG. 23 in Lonnberg shows the configuration of a large-diameter cyclone. The pressure cyclone consists of a cyclone with steam/pulp inlet and a steam outlet, a jacket scraper, a plug screw feeder and a counter-pressure device in the bottom. The surplus steam from the refiner blows the pulp to the top of the pressure cyclone, where it is fed in tangentially under pressure. The pulp and steam are separated by the combined effect of centrifugal and gravity forces. The steam goes upwards in the center of the cyclone and out to a heat recovery system. A scraper prevents pulp from getting stuck on the inside of the jacket. In the bottom of the cyclone a discharge screw feeds the pulp to a latency tank. The pulp plug and the counter-pressure device seal against the steam pressure in the cyclone.
WO 2010/001097 discloses a cyclone separator for separating particles from a mixture of gas and particles, said cyclone separator comprising: a separation chamber in which the particles are separated from the gas; an inlet configured to provide the mixture of particles and gas to the separation chamber; a reverse flow gas outlet positioned to receive a portion of the gas, from which particles have been separated, from the separation chamber, the direction of this portion of the gas having been reversed in the separation chamber; and a unidirectional flow gas outlet positioned to receive another portion of the gas, from which particles have been separated, from the separation chamber, the direction of this portion of the gas not having been reversed in the separation chamber.
Steam explosion is a well-known pre-treatment process for lignocellulosic feedstocks, in which the ligno-cellulosic feedstock is first subjected to a hydrothermal treatment in the presence of steam at high temperature and pressure, followed by rapid release of the pressure applied to the feedstock to produce an explosive disruption of the lignocellulosic structure. Thereby, the feedstock is inserted in a pressurized reactor, wherein the pressure is usually obtained by inserting steam in the reactor at a temperature which can be about 200° C. Steam reactor pressure can be as high as 20 bar, thereby far exceeding the pressure applied to the wood feedstock in chemical pulping process. A mixture of ligno-cellulosic feedstock and fluid comprising water in liquid or vapor form is removed from the pressurized reactor through a feedstock outlet and introduced in a blow cyclone at about atmospheric pressure through a blow line. Due to the change of the pressure applied to the feedstock, the water entrapped in the feedstock cells is subjected to a rapid expansion, causing the expansion of the feedstock cells until reaching in some cases the explosion of the cells themselves. Therefore, in a steam explosion process the pressure applied to the feedstock is released as quickly as possible, by suitably designing the configuration of the blow line.
Consequently, the solids/fluid mixture is accelerated through the blow line by the difference of pressure between the pressurized reactor and the blow cyclone, and at the entry in the blow cyclone it may attain a velocity which is close to the sound speed. The velocity of the solids/fluid mixture is far exceeding the velocity attained by the pulp at the entry of the blow cyclone in a pulp process.
The solids/fluid mixture is typically introduced in the blow cyclone tangentially or almost tangentially, which means that its velocity direction at the inlet of the blow cyclone forms a low angle with the impact point or area on the blow cyclone wall. Differently from the pulp process, in a steam explosion process the solids in the blow cyclone behave as bullets striking the blow cyclone wall.
When used in a steam explosion process, a blow cyclone designed for a pulp process is therefore subjected to abrasive erosion and failure due to perforation of the cyclone wall in a short operating time, which can be in the order of a few days. Besides the repair costs, frequent downtime cycles have dramatic consequences on the process performance and costs, especially in an industrial plant operated continuously.
There is therefore the need for a blow cyclone which can be used without failing and being damaged when solids/fluid mixture is introduced at a high velocity.